I was thinking about making a focused using an inner tube 1.25" threaded going into a 1.375" threaded tube. This is a helical focused, which is very nice for precise focusing. Then put the outer tube in a larger tube that slides in and out, held by a thumbscrew, to get coarse focus. The problem is that the larger taps and dies are hundreds of dollars each. Does someone rent items like that? Obviously if you break it you bought it, but that is a strong case for being very very careful.
2" helical focusers used to be fairly common--I put one on an 8" f/8 Newtonian reflector I built back in '96. I fabbed a 2"/1 1/4" adapter from phenolic to insert in the helical; I mounted a small tab on the top for coarse sliding adjustment, then used the helical itself for fine adjustment. It works quite well. The helical focusers were inexpensive. Maybe you should look around on the webz to see if they're still available. Probably much cheaper than trying to fabricate the whole thing, reinventing the wheel, you know?
ReplyDeleteAuto parts stores often rent expensive specialty tools, and might know where to rent a tap and die set even if they don't rent them themselves.
ReplyDeleteEqually, a machine shop might either rent one our or just do it for you for a small fee?
Generally speaking, threads that large are cut on a lathe, or thread milled on a CNC machine. Perhaps a local shop or makerspace could do just the threads for you?
ReplyDeleteMy first thought is that a single step using an expensive tool that you won't use for anything else might be farmed out to a shop that already has and uses the tool.
ReplyDeleteYou might also check Home Depot. They rent tools including plumbing and they own Ridgid Tool which makes larger tap and die sizes.
ReplyDeletePeople tapping/cutting threads at this size will usually do it on a lathe. You might be able to find a machine shop in your area that can handle it for you.
ReplyDeleteThough I can't quite picture what you're talking about.